The present invention is generally directed to material handling vehicles and, more particularly, to an automatic guided vehicle that is capable of automatically loading and unloading a transport, including loads near the end of the transport with minimal interference, even when the transport floor is vertically offset from or angled relative to the floor of the loading bay or a dock ramp extending between the transport floor and the loading bay floor.
Automatic guided vehicles (AGVs) are used throughout the material handling industry to transport loads. The term AGV is commonly used to refer to robust vehicle designs having any of a number of available automated guidance systems. Automatic guided carts (AGCs) is a term commonly used to refer to a less robust vehicle used for similar but less complicated applications. Throughout this application, including the claims, the term AGV shall mean and include both AGVs and AGCs, as well as any other vehicle that is automatically guided.
Current lighter duty AGV designs generally include a frame with swivel castors located at the four corners of the frame. Other features may include a drive wheel assembly and rigid castors for directional control of the cart. In one current design, two rigid castors are fixed to the frame and located approximately midway between the swivel castors on each side of the cart frame. The two pair of swivel castor axes and the rigid castor axis are generally parallel to each other. The steerable driving unit is attached to the cart frame, generally by way of a plate that is hinged and spring loaded from the cart frame to ensure that the steerable drive wheel maintains adequate traction with the support surface. In another embodiment, a fixed drive wheel propels the AGV, and a steerable castor wheel directs the movement of the AGV. Heavy duty AGV designs generally include a heavy duty frame and at least three wheels, with at least one of the wheels being a drive wheel and at least one wheel being a steering wheel directed by a guidance system. Many of these AGV designs are similar to existing vehicles for moving loads in a manufacturing or distribution setting but are automatically guided.
An AGV includes a guidance system that controls its movement. Known guidance systems in use today include wire guidance, laser guidance, magnetic tape guidance, odometry guidance, inertial guidance, and optical guidance, and each have their own associated positives and negatives. For example, inertial guidance is susceptible to tracking errors, where the travel distance and direction measured by the AGV differs from the actual distance and direction of travel. Though they can be minimized, tracking errors may compound over long travel distances and the system must adjust for these errors, for example, by utilizing waypoint reference markers (magnetic paint, Radio Frequency Identification (RFID) tags, etc.) along the designated path.
Laser guidance systems use special markers that the AGV senses and uses to control its travel. This type of system is susceptible to obstruction of markers and, most notably, requires markers to be present in any environment of travel. If the path of the AGV is modified, the markers must be physically moved. Further, an AGV with this type of guidance system can only travel in areas that have these special markers, which, in the context of this invention, require that any transport to be loaded or unloaded include markers.
One difficulty associated with the automatic loading and unloading of a transport is the variable position of the transport in relation to the loading dock. Transports are usually positioned manually; for example, by a driver in the case of a truck. This manual positioning results in an unknown variability in the position of the transport. As a driver positions a transport, such as a trailer at the loading dock, he or she may be unable to perfectly square the trailer with the dock door. This will leave the trailer at a skewed angle in reference to the dock door. Since the angle is unknown and can vary at each positioning at the dock, an AGV cannot effectively guide and deliver loads in the trailer, unless the skew is adjusted or the AGV has the capability of detecting and compensating for this trailer skew. The prior art has addressed this problem by using skid plates to position the transport in relation to the loading docks, however this is a costly and inefficient process. The trailer may also be positioned offset from the optimal position relative to the dock door. In loading wider loads by AGVs, an offset as little as one inch may cause problems during the loading process.
The transport is generally positioned within a transport loading area outside of a loading bay door for loading. Many variances in the positioning of the transport, as well as between the transport and the loading dock, may cause difficulties in the AGV loading the transport and, in particular, the end of the transport. The difficulty associated with automatic loading and unloading of a transport is that the AGV must be able to overcome the difference in height between the transport and the dock. Different types of transports, as well as different styles of the same transport, will vary in height. Furthermore, the height of a particular transport is not static; as the transport is loaded the suspension will compress, resulting in a change in the height of the transport. In order to allow robust operation, the AGV must be able to operate with varying transport height and, therefore, varying height differences between the transport and dock. The variance in height may cause the load to contact the roof of the transport or the lip of the roof at the entrance or threshold of the transport. Any contact between the load and the transport will cause problems in the loading of the transport. The prior art has addressed this problem by using hydraulic or other types of jacks to stabilize and level the transport; however, this is another costly and inefficient process.
The variability in position of the transport may prohibit the automatic loading of the transport, and almost certainly will reduce its efficiency. For example, the most efficient loading process positions the loads as closely to each other as possible, and any variability in the expected position of the transport will tend to increase the separation of the loads.
Other problems may also occur, which cause differences between the actual location of the AGV and the expected position, as determined by the guidance system. One cause of such problems is slick surfaces on which the AGV travels, including the loading bay floor, dock ramp, and transport floor. As many transports are commonly semi-truck trailers, they may be used to haul a variety of products which may spill or leak slippery substances. The transports are also exposed to many other environmental conditions, including moisture that condenses on the floor of the transport and, in some cases, forms a frost or ice layer on the transport floor. The transports may be used at a variety of facilities and, in some circumstances, loading equipment may leak slippery substances such as oil, hydraulic fluid, and other fluids onto the transport floor. As the AGV loads and unloads various transports, these substances may be transferred by the AGV wheels to the dock ramp and loading bay floor. As the floor of the transport and the dock ramp may commonly be at an angle offset from level, it is easy for the AGV to experience wheel slippage, whether caused by water, ice, oil, or other substances. Any wheel slippage may cause the actual position of the AGV to vary from the expected position determined by the AGV.
As expected, any differences between the expected position and actual position may cause errors in placement of loads, undesirable contact of the AGV or load with transport walls, or future guidance errors. Some systems have been developed to ensure that the load or AGV does not contact the transport side walls. One such solution has been to continuously measure the distance of the AGV from the walls and constantly adjust to keep the AGV centered between the walls. One problem with this system is that it may slow down the loading and unloading of transports, as the AGV must constantly measure and adjust for any variations.
A dock ramp may compensate for any variation between the height of the transport floor and the loading bay floor. The transition between the two may require a steep incline or decline between dock and transport, which can cause guidance difficulties and end-of-trailer loading difficulties. For example, an AGV that uses a laser guidance system may lose the target as it moves up an incline or down a decline, due to the fact that the laser will be pointing either above or below the target. The difficulty with end-of-trailer loading for the above described transport and transport loading area facilities is that if the AGV is not at an equal angle to the transport floor, such as the majority of the AGV being situated on the dock ramp or loading facility floor, it may be difficult for the AGV to lower its load to the transport floor and then easily remove its forks from the pallet pockets. More specifically, if the transport floor is not aligned both vertically and angularly with the loading facility floor or dock ramp, it may be difficult to withdraw the forks from the pockets on the pallets as the tip of the fork engages one of the top and bottom, and the portion of the fork nearest to the AGV engages the other of the top and bottom. Therefore, when the AGV attempts to back out of the transport after dropping the last load, the last load may be pulled backwards with the AGV. The angle of the two supporting surfaces may become greater as the transport is loaded. As more loads are added to the transport, the suspension becomes compressed. As the suspension compresses, the height of the trailer lowers, thereby increasing the angle between the transport floor and the supporting surface of the AGV. The dock ramp is typically formed from steel and may become slippery, causing errors with the guidance system.